Process for producing bonded mica



- i irentedoa, 10,1950 I 2524 1932 1 fi l; 'i fv lq 1 :J gnu e, I} eatinghcnse Electric Corporation, East Pith mul ,segee jnm qv i im m-12,1946 Serial Nut-sanct eldefects in-th e render it I duclng bondedgmica 5 0i substantially uniform densityvandgthe product-derived from the Iirocess. -sulation requirements are high. o In preparing ,pressed laminated products; from ,.,;It,has beensuggested tosecure a moreuniform mica andah'indembyaany.:knowncprocess it has distribution otrthe'binder m mica'compositions been the experiencein the art that the distribu-" =by;interposing=a pad ofasbestos cloth :or: the like a tion of=;the binden:with'respect-tothemlcaflakes app1ied-,=' for instance, between: one 'surfa'ce'of the is; relatively'momuniform,that is, ':the binder. is v 'micawand the press platen during the pressing of 4 present. in: widely adifierent amounts at different the laminate. From experience it has been found in the material.- :Often the binder collects that,,while some improvement'is efiected by-the qinthe formiof a :small pool at one or xnore'points, 'use of the. pads-,"the binder is still distributed in mwhereas 'atwother points the amount of binder a non-uniformmanner-"throughout the material greatlydeficient.'- Whenbonded' mica-charand failures and-defects'in electrical apparatus -acterized=bynthisrlack'of'iuniformity is employed vemployingsuchinsulatiorroceur frequently.

t' as electrical insulation-between commutator seg- 1 It hasfrtlso' been found that the compression set vgments,=for'example;:due to'the high pressures" and or compressibility of mica produced by pr-ior art temperatures met inservice, premature flow tends 7 *methods' at pressures of 1000- pounds per "square 'i-to occur where thebinder has collected'in a pool inch while at'temperatures 'of150" C; is relatively -r-or is present in great: excess; Once flow has high and may' vary considerably from sample to -'started;tho commutator segments are forced to sample "of material nroduced'under the same conreadjust themselves,'- and misalignment takes ditions of manufacture'fiThisi'entlers-it difllcult ;-tplace and'exce's'sive pressures build up.'-' In some both' to'design'and to manufacture electrical ap- :cases, the mica willskid or squeeze out from beparatusemployin'g bonded mica insulation under or mechanicali failurethereof; P v "bonded mica insmatim;

thicknessiaof the mica flakes at v'aiious points 3 'tween the commutators at -the points 'whereex- 'press'ure'g-or for'instancepin comniutators. A lce'ssbinder 'is presenti' Mis'alignment"of the considerable numbefof failures"inmanufaeture, rco'mmutator willresult in"roughness and'other test and service of electricalf i aratus produced defects which lead to Door operation of'the elecwith ="mipa insulation F inay be tracdf'tolack; of Jtrical:apparatus andoften times-cause'elect'rical uniformity in"the mechan al brop erti of the In E reparin'g compositions fromin'ic" and a m omeettums iiiten on isftdprovide jfor Lbinder; it-zis thefpractiee to' form lay'ers 'o'f mica "consolidating' mica flakesj tl'i'apinder under a flakes :mech'anically or by hand in combination CSiSilIf --in"thelayer "-th I I I distributed in' i'ic manner that h gh-spots and 310W "spots ar the laj erf()f'fiiiiita onfl'ake's so proiliide' ubj'ected' to-compression in conventional'ipres h vii'fg metal plat nsin or- 4 der to con's'oliilate the "pressures I v 1mm pressureoir'erith'e entire layer The ow *onthe other liaridareisuhj ected topijessure sider'ably fibelo 'average unit pressure a -spotsi Furthe'rnioi'e the mica attlie sis nece'ssalrilyi' subject to crushing Fig. 3 is a schematic view of a process for producing formed mica insulation.

According to the present invention layers or bodies of mica flakes treated with a fluid or a flowable binder are subjected to a substantially uniform unit pressure over the entire surface thereof. More specifically-the body '0! mica flakes and flowable binder applied thereto is sub- Jected to pressure applied through an elastic member, said elastic member being subjected to- The elastic member will con-' fluid pressure. form to and apply a substantially uniform pressure regardless of the shape or form of the surface of the mica body to which it is applied.

Referring to Fig. l of the drawing, there is illustrated an apparatus l for carrying, out the process. The apparatus comprises a substantially rigid platen or base l2 provided with hollow coils or tubes 14 in which a heating fluid or a cooling fluid may be circulated to secure predetermined temperatures. It will be appreciated that the tubes I 4 may contain an electrical heating element or other heating means if desired. Disposed above the platen i2 is a movable press head I 6 having a cavity [8 disposed along its underside. An elastic member 20 is applied to the head It to close the cavity iii. A conduit 22 is disposed in the head l6 for introducing steam or hot water or other suitable fluid under pressure into the cavity i8. Cooling water may be introduced into the cavity by the conduit 24 and vented through the conduit 26. The elastic member or diaphragm 20 may be secured to the sides of the press head l6 preliminary heating with or without vacuum treatment to remove essentially all the volatile solvent from the binder prior to being placed in the apparatus Hi. The uncompacted layer of mica flakes with the binder distributed therein will be found to be heterogeneously disposed and characterized by relatively high spots and low spots due to natural variation in distribution. Hand laid sheets may be somewhat more uniform but are still subject to unavoidable irregularities so that relatively high and low spots are present.

The fluid binder is preferably, though not neces-- sarily, a heat curable or thermosetting composition. Examples of such compositions are shellac, mixtures of shellac with gum elemi or copal gum or other natural resin, alkyd resins, or alkyd resins combined with a vinyl or vinylidene monomer, and other resinous compositions. In some cases, inorganic compositions capable of curing to a relatively thermoset state may have been applied to the mica flakes. Solutions of the binder in a volatile solvent are commonly employed. After heat treatment to remove the solvent, the binder in the mica layer is not a true fluid composition but is capable of fluid flow when heated and pressure is applied thereto.

The mica layer 30 with the uncured flowable 1 contact with the upper surface of the mica layer 30 and flxing the head in this position. Steam is then admitted at a pressure of from about 20 to 100 pounds per square inch through the conduit 22, thereby forcing the elastic member 20 at a corresponding unit pressure against the upper surfacecf the'mica layer. Heat'from" the steam is transferred through the elastic member to the mica layer 30. Simultaneously, steam or hot water may be admitted to the tubes H in the platen l2 for heating the layer of mica flakes from the underside. Through the agency of the elastic member the fluid pressure is distributed substantially uniformly over the entire layer 30 regardless of high or low spots in the layer. The high spots will not be subjected to crushing nor will they be subject to unit pressures differing from the unit pressure at low spots and there will be no tendency to drive the binder to lower spots. Therefore, the binder will be substantially uniformly distributed, with respect to the mica throughout the entire layer 30. Pools or puddles of binder will'not form at any point. The pressure and heating may be continued for a period of time of from a few minutes to one hour or more. With heat curable binders the temperature selected and the time of application of heating and pressingwill be selected in accordance with the type of product desired. A fully cured product may be desired in some cases and temperatures of as much as 700 C. and times of'one hour for the treatment may be employed. Where a partially cured sheet of mica composition is desired, for example, for reforming into some complex shape, the treatment may be carried out only forafewminutes.

At the end of the predetermined pressing period, the admission of steam or hot water to the conduit 22 is terminated and cooling water or the like will be admitted to the conduit 24. The cooling water after circulation inthe cavity l8 may be vented through the conduit 26 until the mica insulation has been cooled down to room temperature or thereabouts.- The press head I6 is then elevated and the mica layer 30 removed from the press.

Referring to Fig. 2 of the drawing, there is illustrated a cross section through the layer of bonded mica 30 resulting from the pressing opmaterial of equal thickness using the same binder as produced by any conventional process known. It will be apparent, however, that for most applications the rough upper surface 36 must be removed by machining to the level shown as 32 to produce a sheet of substantially uniform thickness with a smooth upper surface. It will be found that the product produced by the apparatus of Fig. 1 must be machined considerably more to secure a periectly'flat sheet than the mica compositionsof the prior art. This is due to the fact that prior art processes tended to drive the binder into the low spots of the layer, thus building up the low spots and crushing the high spots, thereby attaining a lesser degree of non-uniformity of the upper surface 36 at the expense, however, of the uniform ty of distribution of binder and the integrity of the mica itelf. For a given size sheet, it is necessary to Per cent Hydraulically pressed 10.8 Conventional pressed-Va" asbestos pad 21 Conventional pressedno pad... 19 Conventional press-M," thick rubber blanket of 60 Shore A hardness 89 Conventional press-3*," thick rubber blanket of 50 Shore A hardness 90 Conventional press% 'f thick sponge rubber blanket of Shore hardness 13.2 Conventional press-V thick rubber blanket of Shore hardness 11 It will be noted that the asbestos pad gives a ,coefllcient of dispersion about the same as that obtained with no pad. This indicates that the high spots are almost equally-over pressed.

The 3 1" and A thick rubber blankets are so thin that they crush almost completely without any substantial flow and this impairs the bond distribution. Therefore, a thin rubber blanket is a detriment rather than a benefit in pressing mica.

The use of an elastomeric pressing blanket of thick or thicker gives results entirely unexpected in view of the negative results obtained from both the a; and rubber blanket. It is thoughtthat an elastomeric blanket about thick is required to secure sufllcient body to secure a true flow of the elastomer thereby rendering theunit pressure constant.

Theproces's'of this invention is most advantageous forf treating mica combined with a heat curable binder which may well be reacted to a relatively thermostat state. The use of hydraulic pressure applied through an elastic membrane also may be beneficial in consolidating mica sheets treated with a thermoplastic binder. In the latter case consolidation will produce a well bonded mica sheet having no crushed mica and free from undesirable bond distribution.

Since certain obvious changes may be made in the above procedure and different embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. In the process of preparing bonded mica, the steps comprising placing a sheet of mica flakes carrying a heat curable flowable binder on a relatively rigid, smooth base, covering the sheet of mica flakes with an elastic member, forcing the elastic member against the sheet of mica flakes in the direction of the base by means of fluid pressure thereby to apply a, uniform pressure and to secure a uniform distribution of the binder relative to the mica, heating the sheet of mica flakes at a temperature suflicient to cure the binder to a non-flowable state, removing the heat cured mica sheet from the base and machining to smoothness the relatively rough surface that 8 has resulted from contact with the elastic memher.

2. In the process of preparing bonded mica, the steps comprising placing a sheet of mica flakes carrying a heat curable, flowable binder on a relatively rigid, smooth base, covering the sheet of mica flakes with an elastic member,

a forcing the elastic member against the sheet of mica flakes in the direction of the base by means of fluid pressure thereby to apply a uniform pressure and to secure a uniform distribution of the binder relative to the mica, heating the sheet of mica flakes at a temperature sumcient to cure the binder, continuing the application of heat and pressure until the binder,- is partially cured suiliciently to be substantially free of fluid flow, removing the partially heat cured mica sheet from the platen, machining to smoothness the relatively rough surface that has resulted from contact with the elastic member, and reforming the partially cured mica sheet under heat and pressure to predetermined shape until the binder is fully cured to a non-flowable state.

3. In the process of preparing bonded mica insulation, the steps comprising forming a body of mica flakes and a heat-curable flowable binder applied thereto, subjectin the formed body to substantially the same pressure over the entire surface while being heated at an elevated temperature suitable for curing the binder, thereby providing a uniform distribution of the binder throughout the compressed body, continuing the application of heat and pressure until the binder is cured to'a non-flowable state, and machining a press, subjecting the formed body to substantially the same pressure over the entire surface while being heated atan elevated temperature suitable for curing the binder, thereby providing a uniform distribution of the binder throughout the compressed body, continuing the application of heat and pressure until the binder is cured to a non-flowable state, and machining the relatively rough surface of the bonded mica product so produced to remove surface irregularities.

5. In the process of consolidating a layer of mica with an applied hardenable binder disposed on a smooth, unyielding base, the step comprising subjecting the layer of mica flakes and the binder applied thereto to a substantially uniform unit pressure ovger the entire layer whereby the binder is substantially uniformly distributed with respect to the mica flakes, hardening the binder, and machining irregularities off the surface of the mica layer that was not against the smooth base.

6. In the process of consolidating a layer of mica with an applied heat curable binder disposed on a smooth unyielding base, the step comprising applying fluid pressure through an elastic member to the layer of mica flakes and the applied binder distributed within the layer to consolidate the layer of mica flakes and binder with the binder being substantially uniformly distributed in the layer, heat curing the binder, and machining irregularities oi! the surface of the mica layer that was not against the smooth base.

'I. In the process of producing bonded mica, the steps comprising preparing a' layer of mica flakes and a flowable thermosettable binder distributed therein, compressing the layer between a relatively rigid flat smooth surface and a yielding resilient surface corresponding in its effect to a fluid diaphragm member, to apply to the layer a substantially uniform pressure over the entire surface of the layer at a value below that sufficient to crush mica flakes, whereby a substantially uniform distribution of the flowable binder with respect to the mica flakes is produced, heating the compressed layer to a temperature suflicient to thermoset the binder, relieving the pressure on the layer thereby producing a bonded mica layer having one flat. smooth surface and the other surface being highly irregular with many projections, and machining the irregular surface to remove the projections whereby a bonded mica layer of great uniformity in physical and electrical properties is obtained.

EARL L. SCHULMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

1. IN THE PROCESS OF PREPARING BONDED MICA, THE STEPS COMPRISING PLACING A SHEET OF MICA FLAKES CARRYING A HEAT CURABLE FLOWABLE BINDER ON A RELATIVELY RIGID, SMOOTH BASE, COVERING THE SHEET OF MICA FLAKES WITH AN ELASTIC MEMBER, FORCING THE ELASTIC MEMBER AGAINST THE SHEET OF MICA FLAKES IN THE DIRECTION OF THE BASE BY MEANS OF FLUID PRESSURE THEREBY TO APPLY A UNIFORM PRESSURE AND TO SECURE A UNIFORM DISTRIBUTION OF THE BINDER RELATIVE TO THE MICA, HEATING THE SHEET OF MICA FLAKES AT A TEMPERATURE SUFFICIENT TO CURE THE BINDER TO A NON-FLOWABLE STATE, REMOVING THE HEAT CURED MICA SHEET FROM THE BASE AND MACHINING TO SMOOTHNESS THE RELATIVELY ROUGH SURFACE THAT HAS RESULTED FROM CONTACT WITH THE ELASTIC MEMBER. 